1. Field of the Invention
The present invention relates to frequency converters and more particularly to such converters which utilize a local oscillator signal and a phase-shifted local oscillator signal.
2. Description of the Related Art
A frequency converter is a circuit which generates a useful output frequency which is the sum or difference of the input frequency and the frequency of another signal, usually derived from a local oscillator. A frequency converter can be used to demodulate a radio frequency signal, such a circuit being sometimes referred to as a mixer. One application for a mixer is as a down-converter of a radio frequency signal from an earth-orbiting satellite. Such converters produce an intermediate-frequency signal containing information of interest by demodulating the radio-frequency signal from the satellite.
One prior art mixer is disclosed in S.A. Maas, "A GaAs MESFET Mixer With Very Low Intermodulation", IEEE Trans., Microwave Theory Tech., Vol. MTT-35, No. 4, April 1987, pp. 425-429. The Maas reference utilizes a field-effect (FET) transistor as a mixer and relies upon filter rejection to achieve isolation of the radio frequency (RF) and local oscillator (LO) signals. So doing requires use of an LO signal which is substantially separated in frequency from the RF signal.
Another prior art mixer is disclosed in S. Weiner, D. Neuf, S. Spohrer, "2 to 8 GHz Double Balanced MESFET Mixer With +30 dBm Input 3.sup.rd Order Intercept", IEEE MTT-S Int. Microwave Symposium Digest, Vol. 11, 1988, pp. 1097-1100. The Weiner et al. mixer uses two 180 degree baluns. This approach provides good isolation between the LO and RF stages but does not inherently provide good port return loss. In addition, the baluns are almost impossible to implement in a monolithic microwave integrated circuit (MMIC).